Avoiding clipping in audio power delivery by predicting available power supply energy

ABSTRACT

A power output circuit supplies an audio power output signal that is adjusted to prevent clipping when needed based on an estimate of available energy from the power supply supplying the power output circuit. The power output circuit may be an audio power output circuit that generates an audio power output signal from samples of an audio program that are stored in a buffer. A processing block determines an energy requirement for producing the audio power output signal from the audio program and adjusts an amplitude of the audio power output signal in conformity with the determined energy requirement and an available energy determined for the power supply so that the audio power output signal is reproduced without clipping of the audio power output signal.

The present application Claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application 62/854,033 filed on May 29, 2019, thedisclosure of which is incorporated herein by reference.

BACKGROUND 1. Field of Disclosure

The field of representative embodiments of this disclosure relates toaudio or other power output methods, circuits and systems that havelimited power supply capabilities and/or constrain the current/powersupplied to one or more power output circuits.

2. Background

Audio output stages that deliver power to an acoustic output transducer,e.g., a micro speaker or loudspeaker are frequently a large consumer ofenergy in energy-limited devices such as mobile telephones and otherpersonal audio devices. If insufficient voltage is available to deliverthe instantaneous audio power output signal at the full linear-scaledvoltage waveform corresponding to the input program material, thendistortion, i.e., “clipping” is the result. Various techniques have beenprovided for preventing such distortion, such as schemes that boost thepower supply voltage momentarily, e.g., so-called “class G” and “classH” amplifiers, so that while momentary energy demand is increased, peaksof the output waveform can be reproduced accurately. In someimplementations, the amplitude of the output waveform is reduced whilethe boosting action raises the power supply voltage.

However, such power-only or amplitude-only solutions only consider apresent state of available voltage and current and do not consider thefuture state of the power supply voltage, leading to inaccuracies.

Therefore, it would be advantageous to provide an improved performancein audio power output circuits, in particular when the audio poweroutput circuits share a power supply with other circuits.

SUMMARY

Improved operation of audio circuits and systems may be accomplished inamplifier/signal processing systems and amplifier circuits and theirmethods of operation.

The methods, systems and circuits supply a power output signal from anoutput circuit. The power output circuit generates the power outputsignal from samples of an input program that are stored in a buffer. Aprocessing block determines an energy requirement for producing thepower output signal from the input program and adjusts an amplitude ofthe power output signal in conformity with the determined energyrequirement and an available energy determined for the power supply sothat the power output signal is reproduced without clipping of the audiopower output signal. The power output signal may be an audio signal andthe input program may be an audio input program. Alternatively, thepower output signal may be one for driving another electromechanicaloutput transducer, such as a haptic feedback device.

The summary above is provided for brief explanation and does notrestrict the scope of the Claims. The description below sets forthexample embodiments according to this disclosure. Further embodimentsand implementations will be apparent to those having ordinary skill inthe art. Persons having ordinary skill in the art will recognize thatvarious equivalent techniques may be applied in lieu of, or inconjunction with, the embodiments discussed below, and all suchequivalents are encompassed by the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system in which techniques according toan embodiment of the present disclosure are practiced.

FIG. 2 is a block diagram illustrating processing within signalprocessing block 20 of FIG. 1.

FIG. 3 is a block diagram illustrating processing within capacitanceestimator 30 of FIG. 1.

FIG. 4 is a block diagram of a system in which techniques according toan embodiment of the present disclosure are practiced.

FIG. 5 is a graph depicting operation of the systems illustrated herein.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

The present disclosure encompasses methods, systems and circuits thatcontrol the generation of an audio power output signal according toenergy measurements associated with a power supply in order to preventclipping or other distortion of the audio power output signal waveform.The techniques use measurements of the power supply output voltage, thepower supply input voltage (e.g., battery terminal voltage), a currentlimit value provided by the power supply and the value of the powersupply output capacitance to calculate the available energy as the sumof the stored energy in the power supply output capacitance and theenergy transfer available through the power supply. The available energyis compared to a calculated energy needed to deliver the output voltagewaveform to the connected audio output transducer(s) based on samples ofan audio program input stored in a buffer that delays the audio inputprogram. If the available energy is insufficient to meet the outputvoltage waveform requirement for the samples in the buffer, theamplitude of the audio input program is reduced to generate an outputvoltage waveform that will not cause a demand exceeding the availableenergy, preventing clipping of the output voltage waveform. The value ofthe power supply output capacitance may be updated via a computationbased on conservation of power between the power supply input, the powersupplied to generate the audio power output signal, and power due toremoval of energy from the output capacitance. The techniques describedherein may be applied to other power output circuits, such as those thatdrive electromechanical devices such as haptic feedback devices.

FIG. 1 shows a block diagram of an example amplifier system and circuit10 in which techniques according to an embodiment of the disclosure arepracticed. A power converter 14 delivers operating voltage V_(SUPPLY) toan amplifier 16, which generates an output voltage waveform 18, which isultimately delivered to an audio output transducer, such as a microspeaker or loudspeaker SPKR. Alternatively, the audio output transducermay be a haptic feedback device such as a linear resonant actuator (LRA)or eccentric rotating mass (ERM). Across the output of power converter14 is an output capacitance C_(O), which represents the total paralleloutput capacitance at the output of power converter 14 including theeffective output capacitance of any switching circuits in powerconverter 14 and any externally connected filter capacitance, etc. Theinput of power converter 14 is shown connected to a battery 12, but thetechniques herein may be applied to any source of power suitable forproviding input voltage/current to power converter 14. A seriesresistance R_(S) represents the series internal resistance of battery 12and associated external circuit resistance, which causes reduction in abattery terminal voltage V_(BATT) and thus the energy/power deliveredfrom battery 12 to power converter 14. In the illustrated system, if theterminal voltage V_(BATT) of battery 12 is reduced with an activecurrent limit provided by power converter 14, the current that may bedrawn by the battery-powered circuits including power converter 14 isreduced, which could cause an eventual reduction in the output voltageV_(SUPPLY) of power converter 14, which is the voltage across outputcapacitance C_(O). The above-described condition can occur when theoutput power delivered by amplifier 16 is greater than the input powersupplied to power converter 14. If voltage V_(SUPPLY) falls below thepower supply voltage needed to reproduce the output voltage waveform atspeaker SPKR, which is generally the amplitude of output voltagewaveform 18 plus a voltage drop due to the output resistance oftransistors generating the output of amplifier 16, then output voltagewaveform 18 will be clipped at the maximum amplitude until the inputsignal falls in amplitude so that output voltage waveform 18 may againbe generated at full peak voltage.

In order to prevent clipping of the output voltage waveform, a signalprocessor 20 provides a processing subsystem that controls the amplitudeof a signal V_(OUT) that is provided to the input of amplifier 16, sothat when signal processor 20 determines that the energy demanded byfull reproduction of input signal V_(IN) is greater than a measure ofavailable energy, signal processor 20 reduces the amplitude of signalV_(OUT), so that clipping of output voltage waveform 18 does not occur.Details of signal processor 20 are omitted for clarity, but will bedescribed in further detail below with reference to the other Figures.The reduction reduces a gain or increases an attenuation applied toinput signal V_(IN), i.e., signal processor compresses the amplitude ofsignal V_(OUT) uniformly with respect to both polarities of signalV_(OUT), so that clipping and asymmetric distortion is prevented. Signalprocessor 20 receives values of battery terminal voltage V_(BATT) frompower converter 14. It is understood that the above-described techniquesmay also prevent non-linear distortion from amplifier 16 that may occurrather than clipping when output voltage V_(SUPPLY) of power converter14 approaches the voltage that output voltage waveform 18 would assumewithout gain reduction by signal processor 20 by including an offset inthe energy demanded to accommodate any additional voltage needed at thepower supply input to amplifier 16. Signal processor 20 requires a valueof output capacitance C_(O), in order to determine the portion ofavailable energy stored in output capacitance C_(O), which may be apredetermined value, but in some embodiments of the disclosure includesa capacitance estimator 30 that determines a value of output capacitanceC_(O) from measurements within the circuit of FIG. 1 and which will bedescribed in further detail below with reference to FIG. 3. Capacitanceestimator 30 also receives the input signals provided to signalprocessor 20, which will be disclosed in further detail below and havebeen omitted from FIG. 1 for clarity.

While the following description is provided with reference to a blockdiagram, it is understood that the description and the calculationsincluded therein are applicable to a process that may be implemented bya digital signal processor executing a computer program productaccording to an embodiment of the disclosure as described in furtherdetail below. Further, the disclosed embodiment is a discrete-timeembodiment that calculates energy values and power values as sequencesbased on sequential values of voltage measurements and input samples.“Continuous” as used herein includes discrete-time sampled processesthat update values periodically as a sequence of values, generally at apredetermined sample rate.

FIG. 2 is a block diagram depicting a process of gain control as appliedto signal V_(OUT) by signal processor 20 in the circuit of FIG. 1. Powerconverter 14 receives power supply input current I_(AMP) and generates apower converter output voltage V_(SUPPLY), which is provided toamplifier 16. A calculation of available energy E_(AVAIL) is performedby summing operation 23, which adds a calculated value of the storedenergy E_(STORE) determined by E_(STORE) calculation block 22A with acalculated value of the maximum energy input that is available to powerconverter E_(IN) calculated by E_(IN) calculation block 22B. A demandedenergy E_(OUT) that would be required to generate the output voltagewaveform 18, at full-scale, over samples x[n] of audio input V_(IN) thatare stored in a buffer 25, is calculated by E_(OUT) calculation block22C, and an energy ratio G_(energy) is computed by dividing availableenergy E_(AVAIL) by energy demand E_(OUT). Since the energy of outputvoltage waveform 18 over time is proportional to the square of thevoltage of output voltage waveform 18, a square root computation 26 isperformed on energy ratio G_(energy) to obtain a voltage ratioG_(voltage), which is a ratio of the desired amplitude of output voltagewaveform 18 to avoid clipping to the full-scale amplitude of outputvoltage waveform based on audio input V_(IN). To avoid expanding outputvoltage waveform 18 beyond unity gain, a ceiling computation 27 isperformed to take the minimum of voltage ratio G_(voltage) and unity:MIN(1, G_(voltage)), so that an output gain is never greater than unity.A gain control block is provided by multiplier 28 which scales thesamples x[n] of audio input V_(IN) by output gain of ceiling computation27 in order to reduce the amplitude of output voltage waveform 18 whenavailable energy E_(AVAIL) is less than energy demand E_(OUT).

The energy calculations performed by energy calculation blocks 22A-22Care based on measurements and provided values. E_(STORE) is not thetotal energy stored in output capacitance C_(O), but is the differencebetween the total energy stored in output capacitance C_(O), and theenergy stored in output capacitance C_(O) when the value of powerconverter output voltage V_(SUPPLY) is equal to that required to producethe instantaneous value of output voltage waveform 18 without clipping.Therefore, E_(STORE) calculation block 22A computes the stored energyas:E _(STORE)=½C _(O)(V _(SUPPLY) ² −V _(OUT) ²),E_(IN) calculation block 22B computes the maximum available input energythat can be drawn by power converter 24 from the input power source froma predefined current limit value Limn and the actual terminal voltageV_(BATT) of the battery or other source supplying energy to powerconverter 24. E_(OUT) calculation block 22C calculates demanded energyE_(OUT) from samples x[n] of audio input V_(IN) by integrating the powercorresponding to the individual samples on a continuous basis, i.e.,E _(OUT) =kΣ _(n) ^(n-1-N) x ²[n]/R _(O),where R_(O) is the load impedance of the load connected to the output ofamplifier 16 and k is any gain or attenuation factor between samplesx[n] and amplifier output waveform 18. Given the above values computedby energy calculation blocks 22A-22C, the ratio of available energyE_(AVAIL) to demanded output energy E_(OUT) is computed and thegain/attenuation factor gain set as described above.

FIG. 3 shows details of computations within capacitance estimator 30within example amplifier system and circuit 10 of FIG. 1. The value ofoutput capacitance C_(O) is estimated using power calculations accordingto the conservation of power within example amplifier system and circuit10 of FIG. 1, during periods when the conservation of power is accurate,which in the disclosed embodiment is during periods when power converter14 is current-limited as will be described below. During and across theabove-described periods, the value of the estimated output capacitanceC_(O) is updated continuously, so that variations in the estimatedoutput capacitance C_(O) are tracked over time and temperature. Thepower P_(CAP) provided from output capacitance C_(O) is calculated byP_(CAP) calculation block 32A and the power P_(IN)[n] provided to powerconverter 24 is calculated by P_(IN) calculation block 32B, and which isgiven by:P _(IN)[n]=V _(BATT)[n]I _(LIMIT)[n].The output power P_(OUT)[n] provided by amplifier 16 is computed byP_(OUT) calculation block 32C, which can be computed as:P _(OUT)[n]=V _(OUT) ² /R _(L),where R_(L) is the load impedance at the output of amplifier 16, andP _(OUT)[n]=V _(OUT) ² /R _(L)=(gain x[n])² /R _(L).A subtraction operation 33B calculates the power that is removed fromoutput capacitance C_(O) by subtracting power converter 14 input powerP_(IN)[n] from output power P_(OUT)[n] and subtraction operation 33Acomputes a power error P_(error) due to error in the estimate of outputcapacitance C_(O), by subtracting the computed power P_(CAP) providedfrom output capacitance C_(O). An accumulator 34 accumulates thecomputed power error P_(ERROR) to generate an energy error E_(error).Since, as described in more detail below, the power converter inputpower P_(IN)[n] is determined from the power supply current limit valueI_(LIMIT), accumulator 24 is enabled for update by a control signalcalculate_enable, which ensures that capacitance estimator is onlyupdated when the input of power converter 24 is current-limited. Energyerror E_(ERROR) is scaled by a scaling operation 35 and adder 36 scalethe computed error by a factor (k+1). The error factor is multiplied bya nominal estimated starting capacitance value c_nominal by a multiplier32, so that a corrected capacitance value C_(corrected) is produced thatcan be used in further computations by E_(STORE) calculation block 22Aof FIG. 2.

The power calculations performed by power calculation blocks 32A-32C arebased on measurements and provided values. The actual output powersequence P_(OUT) is computed by P_(OUT) calculation block 32C from inputsamples x[n] and the load resistance at the output of amplifier 16, asdescribed above. The input power sequence is computed as described aboveby multiplying the current limit value LIMIT by the terminal voltageV_(BATT) of the battery or other input power source. The power providedby draining energy from output capacitance C_(O) is computed as:

${{P_{CAP}\lbrack n\rbrack} = {{{V_{SUPPLY}\lbrack n\rbrack}I_{CA{P{\lbrack n\rbrack}}}} = {V_{SUPPLY}{C_{O}\lbrack n\rbrack}\frac{d{V_{SUPPLY}\lbrack n\rbrack}}{dt}}}},$which can be computed from finite differences between the sample valuesof power supply output voltage V_(SUPPLY) and the instant estimate ofoutput capacitance C_(O): C[n].

Referring now to FIG. 4, a digital signal processing system is shown,which can be used to implement the techniques of the present disclosure.A digital signal processor (DSP) 42 (or a suitable general-purposeprocessor) executes program instructions stored in a non-volatile memory44 and that form a computer-program product in accordance with thepresent disclosure. DSP 42 receives samples of the audio input signalV_(IN) at an input INPUT from a program source, such as a CODEC. DSP 42also receives samples of output voltage V_(SUPPLY) of power converter 14from an ADC 41B, which serves as a voltage measurement circuit, and thevalues of current limit Limn and battery terminal voltage V_(BATT) ofbattery 12 from power converter 14. A digital-to-analog converter (DAC)43 receives output values corresponding to the processed amplifieroutput signal V_(OUT), which represent audio input samples that havebeen processed according to the processes described with reference toFIG. 2 and FIG. 3, to prevent clipping or other distortion due toinsufficient energy available from power converter 14 and outputcapacitance C_(O). DAC 43 provides output signal V_(OUT) to an amplifier46, which generates output voltage waveform 18. Alternatively, DSP 42may provide samples to a pulse-width modulator (PWM) (class-D) typeamplifier or generate PWM signals directly provided to switchingcircuits that are supplied with supply voltage V_(SUPPLY) provided bypower converter 14 and output capacitance C_(O).

Referring to FIG. 5, a graph shows an example operation of the amplifierdescribed above with reference to FIGS. 1-4. The available energyE_(AVAIL) is shown occasionally dropping below the demanded energyE_(OUT) and the value gain is lowered by the action of theabove-described processing for these intervals so that clipping of theoutput voltage waveform 18 does not occur.

As mentioned above portions or all of the disclosed process may becarried out by the execution of a collection of program instructionsforming a computer program product stored on a non-volatile memory, butthat also exist outside of the non-volatile memory in tangible forms ofstorage forming a computer-readable storage medium. Thecomputer-readable storage medium may be, for example, but is not limitedto, an electronic storage device, a magnetic storage device, an opticalstorage device, an electromagnetic storage device, a semiconductorstorage device, or any suitable combination of the foregoing. Specificexamples of the computer-readable storage medium includes the following:a hard disk, semiconductor volatile and non-volatile memory devices, aportable compact disc read-only memory (CD-ROM) or a digital versatiledisk (DVD), a memory stick, a floppy disk or other suitable storagedevice not specifically enumerated. A computer-readable storage medium,as used herein, is not to be construed as being transitory signals, suchas transmission line or radio waves or electrical signals transmittedthrough a wire. It is understood that blocks of the block diagramsdescribed above may be implemented by computer-readable programinstructions. These computer readable program instructions may also bestored in other storage forms as mentioned above and may be downloadedinto a non-volatile memory for execution therefrom. However, thecollection of instructions stored on media other than systemnon-volatile memory described above also form a computer program productthat is an article of manufacture including instructions which implementaspects of the functions/actions specified in the block diagram block orblocks.

In summary, the instant disclosure discloses a power output circuit thatsupplies a power output signal that is adjusted to prevent clipping whenneeded based on an estimate of available energy from the power supplysupplying the amplifier. The power output circuit generates the poweroutput signal from samples of the program that are stored in a buffer. Aprocessing block determines an energy requirement for producing thepower output signal from the program and adjusts an amplitude of thepower output signal in conformity with the determined energy requirementand an available energy determined for the power supply so that thepower output signal is reproduced without clipping of the power outputsignal. The system provides a power output signal to an outputtransducer based on a program and includes a power supply, a buffer forstoring samples of the program, a power output circuit that generatesthe power output signal from the samples of the program, and aprocessing subsystem that determines an energy requirement for producingthe power output signal from the program and adjusts an amplitude of thepower output signal in conformity with the determined energy requirementand an available energy determined for the power supply so that thepower output signal is reproduced without clipping of the power outputsignal. The program may be an audio program and the power output signalan audio power output signal. Alternatively, the power output signal maybe a signal for driving another electromechanical output transducer,such as a haptic feedback device.

The processing subsystem may determine an energy available from thepower supply circuit by determining a stored energy in an output storagecapacitance of the power supply, determining an input energy availableto an input of the power supply, and combining the input energyavailable to the input of the power supply circuit and the stored energyin the output storage capacitance to determine an energy available tothe power output circuit. The processing subsystem adjusts the amplitudeof the power output signal in conformity with the determined energyavailable to the power output circuit. The processing subsystem maycalculate the energy requirement based upon the samples of the inputsignal stored in the buffer and compare the energy requirement to theenergy available to the power output circuit to determine a gain orattenuation value to apply to the power output signal. The processingsubsystem may also estimate a capacitance value of the output storagecapacitance and determine the stored energy in the output storagecapacitance using the estimated capacitance value. The processingsubsystem may compute the capacitance value from conservation of a powersupplied from the output storage capacitance, a power supplied to theinput of the power supply, and a power supplied to the power outputcircuit. The system may further include a voltage measuring circuit formeasuring a voltage across the output storage capacitance and determinethe power supplied from the output storage capacitance from the computedcapacitance value and changes in the measured voltage. The processingsubsystem may compute the power supplied to the input of the powersupply from a current limit value of the power supply and from a voltageacross the input of the power supply while the input of the power supplyis in a current-limited condition, and may compute the capacitance valueonly while the power supply is in a current-limited condition. Theprocessing subsystem may further determine an error in the estimatedcapacitance value by subtracting the power supplied to the input of thepower supply and the power supplied from the capacitor, according to theestimated capacitance, from power values representing power delivered tothe power output circuit. The processing system may then accumulate aresult of the subtracting to produce an energy error value, and updatethe estimated capacitance value in conformity with the energy errorvalue. The processing subsystem may update the estimated capacitancevalue so that variation in the capacitance of the output storagecapacitance is modeled over time and compensated for when determiningthe energy available from the power supply.

While the disclosure has shown and described particular embodiments ofthe techniques disclosed herein, it will be understood by those skilledin the art that the foregoing and other changes in form, and details maybe made therein without departing from the spirit and scope of thedisclosure. For example, the techniques of the disclosed embodiments maybe combined with other compression algorithms such as loudness contoursto provide a combined reduction in volume when needed due to the powerconstraint or for signal enhancement.

What is claimed is:
 1. A method of preventing clipping in a power output circuit that provides a power output signal, the method comprising: determining energy available from a power supply that supplies the power output circuit by determining a stored energy in an output storage capacitance of the power supply, determining an input energy available to an input of the power supply, and combining the input energy available to the input of the power supply and the stored energy in the output storage capacitance to determine an energy available to the power output circuit; calculating an energy requirement for producing the power output signal; and adjusting an amplitude of the power output signal in conformity with the determined energy available to the power output circuit so that the power output signal is reproduced without clipping of the power output signal.
 2. The method of claim 1, wherein the power output circuit is an audio power output circuit for supplying an audio power output signal to an electro-acoustic output transducer.
 3. The method of claim 1, further comprising buffering samples of an input signal that is reproduced by the power output circuit to provide the power output signal in a buffer, and wherein the calculating an energy requirement computes the energy requirement based upon the samples of the input signal in the buffer.
 4. The method of claim 1, further comprising comparing the energy requirement to the energy available to the power output circuit to determine a gain or attenuation value to apply to the power output signal.
 5. The method of claim 1, further comprising estimating a capacitance value of the output storage capacitance, and wherein the determining the stored energy in the output storage capacitance of the power supply determines the stored energy using the estimated capacitance value.
 6. The method of claim 5, wherein the estimating a capacitance of the output storage capacitance computes the capacitance from conservation of a power supplied from the output storage capacitance, a power supplied to the input of the power supply and a power supplied to the power output circuit.
 7. The method of claim 6, further comprising measuring a voltage across the output storage capacitance, wherein the power supplied from the output storage capacitance is determined from the estimated capacitance value and changes in the measured voltage.
 8. The method of claim 7, wherein the power supplied to the input of the power supply is computed from a current limit value of the power supply and from a voltage across the input of the power supply while the input of the power supply is in a current-limited condition, and wherein the estimating a capacitance of the output storage capacitance computes the capacitance only while the power supply is in a current-limited condition.
 9. The method of claim 6, further comprising: determining an error in the estimated capacitance value by subtracting the power supplied to the input of the power supply and the power supplied from the output storage capacitance, according to the estimated capacitance value, from power values representing power delivered to the power output circuit; accumulating a result of the subtracting to produce an energy error value; and updating the estimated capacitance value in conformity with the energy error value.
 10. The method of claim 9, wherein the determining an error and the updating are performed so that variation in the capacitance of the output storage capacitance is modeled over time and compensated for in the determining of the energy available from the power supply.
 11. A system that provides a power output signal to an output transducer based on a program, the system comprising: a power supply; a buffer for storing samples of the program; a power output circuit that generates the power output signal from the samples of the program; and a processing subsystem that determines an energy requirement for producing the power output signal from the program and adjusts an amplitude of the power output signal in conformity with the determined energy requirement and an available energy determined for the power supply so that the power output signal is reproduced without clipping of the power output signal, wherein the processing subsystem determines the available energy by determining a stored energy in an output storage capacitance of the power supply, determining an input energy available to an input of the power supply, and combining the input energy available to the input of the power supply and the stored energy in the output storage capacitance to determine an energy available to the power output circuit, and wherein the processing subsystem adjusts the amplitude of the power output signal in conformity with the determined available energy.
 12. The system of claim 11, wherein the power output circuit is an audio power output circuit for supplying an audio power output signal to an electro-acoustic output transducer, and wherein the program is an audio program.
 13. The system of claim 11, wherein the processing subsystem calculates the energy requirement based upon the samples of the program stored in the buffer.
 14. The system of claim 13, wherein the processing subsystem compares the energy requirement to the energy available to the power output circuit to determine a gain or attenuation value to apply to the power output signal.
 15. The system of claim 11, wherein the processing subsystem estimates a capacitance value of the output storage capacitance and determines the stored energy in the output storage capacitance using the estimated capacitance value.
 16. The system of claim 15, wherein the processing subsystem computes the capacitance value from conservation of a power supplied from the output storage capacitance, a power supplied to the input of the power supply, and a power supplied to the power output circuit.
 17. The system of claim 16, further comprising a voltage measuring circuit for measuring a voltage across the output storage capacitance, wherein the power supplied from the output storage capacitance is determined from the computed capacitance value and changes in the measured voltage.
 18. The system of claim 17, wherein processing subsystem computes the power supplied to the input of the power supply from a current limit value of the power supply and from a voltage across the input of the power supply while the input of the power supply is in a current-limited condition, and wherein the processing subsystem computes the capacitance value only while the power supply is in a current-limited condition.
 19. The system of claim 16, wherein the processing subsystem further determines an error in the estimated capacitance value by subtracting the power supplied to the input of the power supply and the power supplied from the output storage capacitance, according to the estimated capacitance value, from power values representing power delivered to the power output circuit, wherein the processing subsystem accumulates a result of the subtracting to produce an energy error value, and updates the estimated capacitance value in conformity with the energy error value.
 20. The system of claim 19, wherein the processing subsystem updates the estimated capacitance value so that variation in the capacitance of the output storage capacitance is modeled over time and compensated for when determining the energy available from the power supply.
 21. A system that provides an audio power output signal to an audio output transducer based on an audio program, the system comprising: a power supply; a buffer for storing samples of the audio program; an audio power output circuit that generates the audio power output signal from the samples of the audio program; and a processing subsystem that determines an energy requirement for producing the audio power output signal from the audio program and adjusts an amplitude of the audio power output signal in conformity with the determined energy requirement and an available energy determined for the power supply so that the audio power output signal is reproduced without clipping of the audio power output signal, wherein the processing subsystem determines the available energy by determining a stored energy in an output storage capacitance of the power supply, determining an input energy available to an input of the power supply, and combining the input energy available to the input of the power supply and the stored energy in the output storage capacitance to determine an energy available to the audio power output circuit, and wherein the processing subsystem adjusts the amplitude of the audio power output signal in conformity with the determined available energy. 